1. Field of the Invention:
The present invention relates to an apparatus for measuring the quantity of particles attached to a surfaces of solid by washing away the particles from the solid surfaces in a liquid and counting the particles contained in the liquid.
2. Description of the Prior Art:
FIG. 4 is a schematic view of a known apparatus for counting the particles attached to the surface of a solid. The apparatus includes a box-shaped measuring tank 1A which is open at the upper portion thereof and which is capable of accommodating a measuring liquid into which is immersed a sample such as a solid on the surface of which particles to be measured are attached, an ultrasonic wave generating device 2A provided at the bottom of the measuring tank 1A for generating ultrasonic waves having a single frequency, and a measuring means 3A, such as a known counter of dust particles in a liquid, disposed adjacent to the measuring tank 1A for counting particles in the measuring liquid contained in the measuring tank 1A. The measuring means 3A has a light source 3a disposed outside the measuring tank 1A and adjacent to one side thereof for radiating light having a constant wavelength, such as monochromatic light, toward the measuring tank 1A, and a light-receiving means 3b disposed at a predetermined angle to the light source 3a for receiving light which has been radiated from the light source 3a and scattered by the particles in the measuring liquid to thereby obtain from the quantity of light which has been received the number as well as the diameter of particles in the liquid contained in the measuring tank 1A.
The principle of counting the particles contained in the measuring liquid by the measuring means 3A will be described below. When a monochromatic light such as a laser beam radiates upon particles in a liquid, the intensity of light scattered by the particles is determined by (1) the diameter of the particles, (2) the wavelength of the light, (3) the index of refraction of light in the particles, and (4) the angle formed by the direction of incidence and the direction of scatter. If items (2) and (4) are fixed, the particle diameter becomes a one-valued function of the intensity of light scattered, and the particle diameter can be therefore unequivocally determined by measuring the intensity of the light scattered, the number of times the scattered light is detected representing the number of particles. In this way, the diameter and the number of dust or particles contained in the measuring liquid can be measured according to the above-described principle.
Next, the operation of the known apparatus for counting particles attached to the surfaces of a solid which is arranged in the above-described manner will be described. After the measuring tank 1A has been filled with a measuring liquid such as a clean liquid chemical or pure water, a sample such as a solid to be inspected is immersed therein, and ultrasonic cleaning thereof is then conducted by the actuation of the ultrasonic wave generating device 2A so as to wash away the particles attached to the surfaces of the sample in the measuring liquid. Subsequently, the sample is removed from the measuring tank 1, and a light beam such as visible light, a laser beam or ultraviolet light then irradiates the measuring liquid from the light source 3a, whereby the light strikes and is scattered by the particles in the measuring liquid, part of the light scattered being received by the light-receiving means 3b. Data representing the intensity of light received by the light-receiving means 3b as well as the number of times the light is radiated from the light source 3a is sent to a data processing device (not shown) where it is used to calculate the number of particles in the measuring liquid on the basis of the above-described principle and thereby to measure the quantity of particles which have been washed away from the sample in the measuring liquid.
Known ultrasonic cleaning of the sample is conducted using ultrasonic waves having a uniform frequency. Since the physical and chemical principles of cleaning may change when the frequency changes, the particles washed into the liquid may vary with the physical properties, the strength of attachment or by the size of the particles attached to the sample, and actual distribution of the particles on the surfaces of the sample may therefore differ from the particle size distribution of the particles measured in the measuring liquid.
Further, the measuring liquid is easily contaminated by dust which enters the measuring liquid from the ambient atmosphere during the process in which the particles attached to the surfaces of the solid are washed away in the measuring liquid or the process in which the particles contained in the measuring liquid are counted, easily generating measurement errors.